In recent years, the mechanism of carbon cycling in an ocean has been progressively clarified in association with global warming. It is known that in the carbonic acid system in the ocean, if two out of four items consisting of the alkalinity of sea water (AT: the difference between the total charge concentration of positive ions in sea water and the total charge concentration of negative ions in sea water), the total carbonic acid concentration (CT: total dissolved inorganic carbon), the carbon dioxide partial pressure pCO2, and the hydrogen ion concentration pH are measured, the remaining items can be calculated. Lately, in particular, it has been desired to measure the carbon dioxide partial pressure in deep sea in order to clarify the carbon cycling mechanism in detail. Non-Patent Document 1 discloses an apparatus for measuring the partial pressure of carbon dioxide according to the related art that can measure pCO2 in deep sea.